Over the years while teaching various Search Engine Optimization (SEO) courses I've been asked many questions about writing SEO copy, or as I prefer it writing for search.
On thing I always stressed is that you first write for your human users (your real audience) and secondly for search engines. This is where an internal battle usually starts about how it should only be for the human users and never for the search engines. To which I reply, fine but without the search engines your human audience is going to be a lot smaller. Once we get over this hurdle, our course conversation usually turns to the issue of duplicate content.
Yes duplicate content and how all work on website/blog has to be original and what that truly means. I try to explain that Google tries strip out all code and just look at the words in the copy. It then compares it to other copy in its index and then has to decided if its original vs. duplicate and if duplicated which one to include in its index. Not a simple task.
While most of us think of duplicated content only occurs when articles are syndicated or if you are incorporating RSS feed content etc there is another and a legitimate reason why you might need duplicate content and why Google should punish you (but they might). This happens for legitimate reasons with big organizations who run country/regional specific websites. For legal reason, marketing or simply management it is very possible for a organization to run multiple unique sites (unique URLs). For example several English language sites target specifically at the USA, Canada, UK, Australia, New Zealand, etc. (same of course is possible for other languages).
The reality is when developing sites in this configuration and to reduce costs, internal documents (white papers, corporate profiles, product pages, etc.) are frequently merely duplicated. So how can you avoid the possible SEO penalties of duplicate content. It's by tweaking the content of these pages. Yes, you can and should adjust the words to reflect the cultural linguistic subtleties of each region, but that usually won't be enough. You should consider at least rewriting the introductory paragraph and a few lines throughout the article. Always take into account the surrounding text on the page (side bars & navigation). Are these unique to each site? Of all the words on the page about 20% should be unique to avoid possible duplication penalties.
One technique, that many people try and fail with, is to merely change some of the connective words. For example changing "also" to "furthermore". These simple changes don't affect the meaning of the content and make life easier. Unfortunately to search engines like Google and Bing their algorithms are programmed to ignore these words along with the basic words "a", "but", "the", etc. (For a detailed list of stop words see: http://www.link-assistant.com/seo-stop-words.html.
So, if you have to re-purpose content for legitimate reason, the re-purpose it, but budget a little time to make a few reasonable changes that will content reasonably unique.